I really like large dogs, and for most of my life, a large dog is what we always had. They are usually loyal and protective of their family, and most of the large dogs have a very mellow disposition, unless they have been taught otherwise.
Even so, if a large dog bites a person, there is a much more likely chance that the person will be seriously injured than if a small fog bites someone. For this reason, I am very careful around any large dog that doesn't belong to me personally.
I watch the dog, and if he seems friendly and approaches me, and the owner is there, then I feel like it is okay to pet him, but if the dog is acting nervous or aggressive at all, then i leave them alone, and don't even try to make friends with that dog.